diff --git a/core/modules/block/tests/src/Functional/BlockTest.php b/core/modules/block/tests/src/Functional/BlockTest.php
index 2e16a24d618f831ecf32a17f8464927e0b0c6c62..e68f58bc4413d095c23cf3c141bd5746db20b687 100644
--- a/core/modules/block/tests/src/Functional/BlockTest.php
+++ b/core/modules/block/tests/src/Functional/BlockTest.php
@@ -244,6 +244,17 @@ public function testBlock() {
     $xpath = $this->assertSession()->buildXPathQuery('//div[@id=:id]/*', [':id' => 'block-' . str_replace('_', '-', strtolower($block['id']))]);
     $this->assertSession()->elementNotExists('xpath', $xpath);
 
+    $pages = [
+      '',
+      '<front>',
+      '/valid-page',
+      'user/login',
+    ];
+    // Test error when not including forward slash.
+    $this->drupalGet('admin/structure/block/manage/' . $block['id']);
+    $this->submitForm(['visibility[request_path][pages]' => implode("\n", $pages)], 'Save block');
+    $this->assertSession()->pageTextContains('The path user/login requires a leading forward slash when used with the Pages setting.');
+
     // Test deleting the block from the edit form.
     $this->drupalGet('admin/structure/block/manage/' . $block['id']);
     $this->clickLink('Remove block');
diff --git a/core/modules/system/src/Plugin/Condition/RequestPath.php b/core/modules/system/src/Plugin/Condition/RequestPath.php
index 796277986b8cbbb18da7c8db77cdb44a1daa87cd..9b4bd474d1a7888aa27adb92ffa6c98dd91edc74 100644
--- a/core/modules/system/src/Plugin/Condition/RequestPath.php
+++ b/core/modules/system/src/Plugin/Condition/RequestPath.php
@@ -112,6 +112,19 @@ public function buildConfigurationForm(array $form, FormStateInterface $form_sta
     return parent::buildConfigurationForm($form, $form_state);
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function validateConfigurationForm(array &$form, FormStateInterface $form_state) {
+    $paths = array_map('trim', explode("\n", $form_state->getValue('pages')));
+    foreach ($paths as $path) {
+      if (empty($path) || $path === '<front>' || str_starts_with($path, '/')) {
+        continue;
+      }
+      $form_state->setErrorByName('pages', $this->t("The path %path requires a leading forward slash when used with the Pages setting.", ['%path' => $path]));
+    }
+  }
+
   /**
    * {@inheritdoc}
    */